This is true, and the same thing happened to me. Or at least I was tested, realized that a gluten free diet would have made the test invalid, and got tested again after consuming gluten for 4 to 6 weeks. That may not have been enough time to cause damage.
I do believe the accuracy once on a gluten free diet depends on the type of test. What you say is certainly true for a biopsy. What kind of test did you have?
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